Presently, communications between land-based systems and earth-orbiting satellites are basically conducted in the C and Ku frequency bands. Consequently, any land-based station which communicates with satellites utilizing either of these two frequency bands would require two different antennas, a feed system wherein the C band feed is offset several beam widths from the bore site of a parabolic antenna and a Ku band feed system upon the bore site of the antenna. Alternatively, the Ku band feed could be offset a plurality of band widths from the bore site and the C band feed provided on the bore site. In either situation, two antennas, as well as two distinct feed systems, must be utilized.
The prior art, in order to save space and money, has proposed utilizing a single dual-beam feed in conjunction with a single parabolic antenna. One such device is described in U.S. Pat. No. 3,665,481 to Low et al. This patent illustrates a multi-purpose antenna employing a dish reflector having plural coaxial horn feeds. The waveguide assembly would include a circular inner tube 16 transmitting an X-band signal of approximately 8.44 GHz, an intermediate tube 18 more than twice the diameter of the inner tube for transmitting an S-band signal of approximately 2.295 GHz, and an outer tube 20 for receiving an S-band signal of a frequency of approximately 2.115 GHz. As shown in FIG. 1 of the Low patent, the low frequency (S-band) waveguide utilizes one conical horn to illuminate the antenna, and the high frequency (X-band) waveguide employs a second conical horn to illuminate the reflector which would block a portion of the radiation of the lower frequency S-band, thereby reducing aperture illumination of the lower frequency band.
The problems caused by this configuration are illustrated in FIG. 1 of the present invention which shows the prior art Low patent. As shown therein, the X-band source of radiation 10 is transmitted through a circular waveguide 16, and the S-band source 12 is transmitted through a circular waveguide 14 which completely surrounds the inner circular waveguide and is concentric with respect to this inner circular waveguide. Since the inner waveguide 16 employs a conical cone 18 and the outer waveguide 14 also utilizes a second conical horn, the conical horn 18 of the inner waveguide 16 reduces the radiation of the outer waveguide 14 and reflects the S-band signal back in the outer waveguide 14, causing the voltage standing wave ratio (VSWR) to be degraded, as shown by the arrow provided in this Figure. While ideally the VSWR of a waveguide should be 1.0, the utilization of the conical horns 18 and 20 of the Low patent might produce an unacceptable VSWR in the range of 1.8 or greater.
U.S. Pat. No. 3,864,687 issued to Walters et al describes a coaxial horn antenna provided with three cylindrical horns 12, 14 and 16 which are progressively sized to provide an inner radiating aperture 18, a concentric intermediate aperture 20 and a concentric outer aperture 22 at the front end of the antenna. The beam width of the frequencies transmitted within these waveguides are controlled by staggering the forward ends of the horns, with the inner horn projecting furthest, the staggering also adding to the isolation. Beam width is reduced as the stagger is increased and can be set to specific characteristics. Although this patent describes a coaxial horn antenna provided with a plurality of waveguides, no recitation is included therein nor is described or suggested by the patent to Low which would optimize the particular geometry of the waveguides such that the diameters of the concentric waveguides would be an optimal length with respect to each other as well as with respect to the particular frequencies which are transmitted therethrough. Although the patent to Low indicated that the outer, low frequency waveguide diameter is more than twice the diameter of the inner, high frequency waveguide, the exact relationship between these two waveguides with respect to the specific frequencies transmitted therethrough has not been addressed.